A spacer is a fluid used in cementing operations and it is pumped between the drilling mud and the cement slurry for replacing said mud. The spacer is essential in order to avoid coming into contact the mud and the cement slurry. This is because the mud and the slurry are chemically incompatible fluids. Further, the spacer helps displace and eliminate the mud from the well by virtue of its special rheological properties and by virtue of its properties encouraging the removal of mud from the walls of the well.
A spacer is composed form an aqueous base fluid in which a particular weighting agent is incorporated to increase the density of the fluid to a desired value, and to increase the erosion effect of the spacer on the mud cake clinging to the walls of the rocks passed through.
The aqueous base fluid must itself have several fundamental properties which are directly related to its composition: particle stability in suspension (anti-settling properties); fluid-loss control; rheology; and compatibility with muds and cement slurries.
Consequently, known spacer base fluids include at least: i) an anti-settling agent; ii) a fluid loss controlling agent; and iii) a dispersing agent.
Each of these agents may be a single product or it may be constituted by a mixture of several products.
Conversely, a single product may perform several of the above-mentioned functions.
The agents are either soluble or else dispersible in water.
Depending on the water available on site and on the geological strata encountered, the aqueous phase may be constituted by fresh water, sea water, brine, or an aqueous phase containing various proportions of dissolved salts, in particular: NaCl (0%-37%); KCL (0%-35%), MgCl.sub.2 (0%-54%); CaCl.sub.2 (0%-75%), on their own or in combination. It is essential that the spacer base fluid retains its properties at all possible concentration of salt(s).
Spacers are used over a temperature range running from surface temperature to the Bottom Hold Circulating Temperature (BHCT) which may reach 150.degree. C., or very rarely 200.degree. C. (geothermal wells).
The term "anti-settling properties" covers the capacity of the fluid to keep the particles of the weighting agent in stable suspension, and to continue doing so throughout the cementing operation, i.e. for a period of time which is 1 h to 4 h long.
Another essential property of a spacer is fluid loss control.
Fluid loss control, measured according to API standards is considered to be good if the fluid loss is less than 50.times.2 ml/30 min, and excellent if the fluid loss is less than 25.times.2 ml/30 min.
The rheology of the composition is also a determining factor.
The important rheological criteria are the flow velocity profile and the capability of providing a pressure drop and/or the capability of readily attaining turbulent flow conditions. Spacer compositions are generally designed either to have a flat velocity profile together with high pressure drops under laminar flow conditions, or else to be suitable for being pumped under turbulent flow conditions in all well configurations.
Finally, given the function of the spacer (i.e. to be pumped between the drilling mud and the cement slurry for removing and replacing said mud in the well annulus) it is absolutely essential for the spacer to be as compatible as possible both with the mud and with the slurry.
Such twofold compatibility is very difficult to obtain. One simple reason, as mentioned above, is that the mud and the slurry are naturally totally incompatible with each other.
Compatibility with the mud and with the slurry is determined by studying the increase in viscosity of a mud/spacer mixture or a cement slurry/spacer mixture varying over the range 0/100% to 100/0% by volume.
Compatibility is considered to be excellent if the viscosity of the mixture at a given shear rate is less than the viscosity of the more viscous component at the same shear rate.
Conventional spacer compositions always contain: i) an anti-settling agent which is generally bentonite or a mixture of bentonite and a cellulose derivative such as carboxymethyl cellulose (CMC); ii) a fluid loss controlling agent which is generally constituted by a hydroxypropylcellulose; and iii) a dispersing agent which is often a polynaphthalenesulfonate.
This constitutes the "base" which is thus made up from cellulose type products, bentonite (solid particles), and a dispersing agent.
The final composition of the spacer is obtained by adding a weighting agent to said base, e.g. barite, and water.
In prior known spacers, the anti-settling agent was either a particulate product such as bentonite or else a polymer such as a cellulose derivative.
However, bentonite can only be used in non-saline water, and in addition the use of bentonite spoils the viscosity by increasing it greatly. Further, bentonite is incompatible with cement which is another severe drawback.
In saline water, bentonite cannot be used and it is replaced by fibrous clays (attapulgite, sepiolite, . . . ), however, these materials do not give the spacer very good anti-settling properties.
The cellulose derivatives which have been used, generally serve both as an anti-settling agent and as a fluid loss controlling agent. However, carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC) are very sensitive to temperature and to salts. They function properly at low temperature and in a non-saline medium. In contrast, for work at higher temperatures the quantity of cellulose derivative that needs to be used, and thus the cost of the process, becomes prohibitive and the viscosity increases too much.
One cellulose derivative which has given very good results is hydroxypropylcellulose (HPC). However, this product is only soluble in warm water and at fairly low salt concentrations.
It can thus be seen that the prior art does not provide a spacer which is compatible both with muds and with cements, whether saline or otherwise, and which simultaneously presents good rheological, fluid loss controlling and anti-settling properties over the entire range of temperature normally encountered in oil field services.
Preferred embodiments of the present invention provide novel compositions which, when used as spacers, have two decisive advantages over known spacers:
compatibility with muds which may be saline or otherwise and/or with cements which may be saline or otherwise, regardless of the salt content of the mud and/or the cement; this is the first time that such total compatibility has been obtained in combination with the three essential properties for a spacer: good fluid loss control, good anti-settling properties, and good rheology. This satisfies a long-felt want in the oil industry; and
a fluid base which is free from solid particles (unlike the bases previously known for spacers, the present base does not contain bentonite).
These surprising results are obtained by a selection of novel anti-settling agents, fluid loss controlling agents, and dispersing agents in the aqueous base.